In current distribution practice, fresh meat is shipped from the slaughterhouse in the forms of large sections comprising as much as a quarter, half or even a whole carcass. These large sections of fresh meat are then cut into smaller weight units called primal cuts and then into subprimal cuts. Upon arrival at the store or central packaging facility the primal or subprimal cuts are further subdivided into retail portions, i.e., roasts, steaks, chops, etc. for ultimate display to the customer.
It is desirable that the final cuts retain their red appearance when on display for prolonged periods of as long as several days. While discoloration of the meat is not per se indicative of inferior quality, consumers overwhelmingly prefer the normal red color of fresh meat. In order to preserve the red meat appearance, and to provide a sanitary package, it has been conventional practice to wrap and heat-seal each cut in plastic film immediately after cutting. Plastic film so utilized must have certain critical physical properties which will insure that the red-meat appearance of the retail cuts will be maintained for the desired period of time under display conditions. For example, it is known that the rate of oxygen transmission through the film should be greater than about 350 cc/100 sq.in./24 hr./atm. at 23.degree. C. If the oxygen transmission is lower than this value, the meat contained in the package will darken in a few hours, a factor that will seriously reduce customer acceptance. On the other hand, as long as the oxygen transmission rate of the meat wrap film is above about 350 cc/100 sq.in./24 hr./atm. at 23.degree. C., the fresh meat will retain its desirable red color. No upper limit exists in any practical sense for the oxygen transmission rate.
In addition to having the above-noted threshold oxygen transmission rate, it is also desirable that the film have a comparatively high rate of carbon dioxide transmission. Carbon dioxide transmission rates of greater than about 1000 cc/100 sq.in./24 hr./atm. at 23.degree. C. are also desirable and are of importance, for example, in the so-called blast freezing of meat with carbon dioxide at temperatures of -70.degree. F. or thereabouts. A high rate of carbon dioxide transmission is necessary to insure that essentially all carbon dioxide retained by the frozen meat can escape when the package is subjected to normal refrigeration temperatures.
Thermoplastic film for wrapping retail cuts of fresh red meat should also possess a proper water or moisture vapor transmission rate (MVTR). It is desirable that the MVTR not exceed about 50 cc/100 sq.in./24 hr. at 100.degree. F. and 90% R.H. If the MVTR is substantially in excess of this value, the meat contained within the package will tend to lose comparatively large amounts of water during display, the weight of the packaged meat will decrease and the cut itself will dry out. The MVTR value must not, however, be too low; otherwise droplets of water may condense and accumulate on the inner surface of the package, a phenomenon commonly known as "fogging". To help prevent fogging of fresh meat-wrap film, it is desirable to select film with an MVTR value of at least 10 g/100 sq.in./24 hr. at 100.degree. F. and 90% R.H., although fogging may be ameliorated by the addition of anti-fogging agents to the resin employed in making the film.
The aforesaid gas transmission rates may be determined by various methods known in the art. For example, oxygen and carbon dioxide transmission rates may conveniently be measured with a Dohrmann Polymeric Permeation Analyzer, PPA-1 (Dohrmann Envirotech Corporation, Mountain View, California). The Dow Cell may also be employed for this purpose, in accordance with ASTM procedure D-1434. The water or moisture vapor transmission rates (MVTR) may be determined in a General Foods Chamber (ASTM procedure E 96) or with a Honeywell Model W 825 Water Vapor Transmission Rate Tester (Honeywell, Inc., Minneapolis, Minnesota) and the rates obtained therefrom converted into General Food units.
Although the preceding gas and vapor transmission properties are of primary importance, it is additionally desirable, for esthetic reasons, that the film be substantially colorless and clear. Clarity is measured by the relative amount of haze present, as determined by ASTM Method D1003-61. For red meat packaging film the haze level should be as low as possible, and preferably less than 1%. The film should also have high gloss and sparkle.
Another important characteristic for food wraps of this type is the property called "cling", which refers to the tendency of the film to adhere to itself or to the container, over or around which it is wrapped. Also, it is important that the film exhibit a high yield strength to prevent stress relaxation which will make the package unsightly. A retail cut which is tightly overwrapped loses customer appeal if the taut film loses its tension and assumes a loose overwrap configuration.
Additionally, the film must have adequate tensile strength as well as high resistance to tearing and to puncture. The film must be heat-sealable, and should not release objectionable, noxious fumes during the heat-sealing operation required after the retail cuts have been wrapped.
For economic reasons, since commercial resins are sold by unit weight, but used by unit area, it is advantageous that resins intended for film wraps have as low a density as possible. In addition, it is also economically advantageous if the properties of the resin are such as to permit the use of film of minimum thickness or gauge, and if such film of minimum gauge can be produced by conventional film-blowing equipment.
It is known in the art that plasticized polyvinyl chloride (PVC) film successfully meets many of the noted requirements for fresh red meat wrap, and large amounts of these resins are consumed yearly for this purpose. Nevertheless, the use of vinyl chloride-based resins for wrapping retail cuts of fresh red meat has certain inherent shortcomings. For example, it has been reported in recent years that the plasticizer, normally comprising 25-50 weight percent of a total plasticized PVC composition, may be extracted by the fatty tissues of the meat and even accumulate in the human body, with possibly unfavorable consequences. Moreover, since the compounding of PVC requires the presence of various stabilizers, for example, barium and cadmium salts and certain organotin compounds, the possibility of the extraction of these materials must also be taken into consideration; as a consequence the list of acceptable stabilizers and other compounds permitted in PVC food applications is quite limited.
More recently, however, great concern has arisen because of growing evidence that a rare form of liver cancer (angiosarcoma) may be attributable to vinyl chloride monomer (Chem. & Eng. News, page 6, Feb. 18, 1974). Experiments have shown that this form of cancer can be induced in rats at vinyl chloride atmospheric concentrations as low as 250 ppm. Prior to packaging and shipping, PVC resins reportedly can contain from 500 ppm to 3000 ppm of vinyl chloride monomer (Chem. & Eng. News, page 16, Feb. 25, 1974).
In addition, when plasticized PVC film is used to wrap retail cuts of fresh red meat, the film is heat-sealed after packaging. Workers performing the packaging are exposed to whatever noxious fumes may be evolved in the heat-sealing operation, and it has recently been claimed that serious respiratory difficulties may arise from such exposure.
Although the packaging of fresh red meat is of principal concern in connection with the present invention, it is also known that PVC film is widely used for in-store packaging of produce such as fresh fruit and vegetables. Whereas such produce does not impose direct contact of the PVC film with fatty tissues, as is the case in meat packaging, many of the other disadvantages of the PVC film are pertinent to produce packaging. Thus, noxious fumes may be evolved in heat-sealing operations and additive residues of potentially toxic materials may be transferred from the packaging film to the contents.
In view of the above-noted difficulties associated with the use of PVC-based resins in food applications generally, it is evident that a need exists for a film-forming resin which, ideally:
(a) equals or surpasses plasticized vinyl film in the above-noted gas and vapor transmissivity, and strength, optical and handling properties;
(b) meets the requirements of (a) without necessitating the addition of a plasticizer which might be extracted by the packaged fresh red meat;
(c) requires no added stabilizer that might also tend to be extracted by the packaged fresh red meat; and
(d) contains no vinyl chloride monomer, or other associated substance capable of causing angiosarcoma.
According to the present invention each of these requirements is met by the provision of certain partially alcoholyzed copolymers of ethylene and vinyl acetate.
So-called copolymers of an .alpha.-olefin, e.g., ethylene, and vinyl alcohol are well known in the art and may, in general, be obtained by alcoholysis or saponification of the corresponding ethylene-vinyl ester copolymers (see, for example, Roland U.S. Pat. No. 2,386,347 granted Oct. 9, 1945). As a matter of fact, however, such copolymers of ethylene and vinyl alcohol are substantially terpolymers of ethylene, vinyl alcohol, and small amounts of residual vinyl ester (up to about 3 weight percent). Pure binary copolymers of ethylene and vinyl alcohol are difficult to obtain by any practical alcoholysis or saponification process known in the art because of the difficulty in reacting the last traces of copolymerized vinyl acetate units.
It is known in the art that substantially fully hydrolyzed ethylene-vinyl ester copolymers of the preceding type may be used as wraps for foods, medicines, and other substances. Thus, it has been disclosed in Gardner et al. U.S. Pat. No. 3,585,177 granted June 15, 1971, that certain lower alpha-olefin/vinyl alcohol copolymers with a residual vinyl ester content below 3% by weight exhibit an oxygen permeability of less than 1.0 cc/100 sq.in./24 hr./cm Hg/mil at 73.degree. C. and a MVTR of less than 8.5 g./100 sq.in./24 hr./mil at 73.degree. C. and 100% relative humidity, provided the alpha olefin content of the copolymer ranges from 5 to 40 weight percent. An ethylene-vinyl acetate copolymer containing from about 75 to 97 weight percent vinyl acetate would be required to make an ethylene-vinyl alcohol copolymer of this composition.
It is further known in the art (see Gerow U.S. Pat. No. 3,595,740 granted July 27, 1971) to make laminar structures containing a barrier layer comprising a copolymer of ethylene and 35 to 85 mole percent of a vinyl ester of a lower aliphatic monobasic acid, at least 85% hydrolyzed, which laminate has an oxygen permeability of less than 3 cc/100 sq.in./24 hr./atm. at 23.degree. C.
It is clear from the foregoing discussion that the so-called ethylene-vinyl alcohol copolymers (containing less than 3% residual vinyl acetate), disclosed by Gardner et al. as packaging materials, as well as the laminate disclosed by Gerow, would be unsuited as wrapping or packaging films for fresh red meat or produce, which require oxygen transmission for good keeping qualities, because the oxygen transmission rate (&lt;3 cc/100 sq.in./24 hr./atm. at 23.degree. C.) is far too low; as pointed out above, wrapping film for fresh red meat should have an oxygen transmission rate of no less than about 350 cc/100 sq.in./24 hr./atm. at 23.degree. C., otherwise the desirable red color of the meat is lost due to oxygen depletion. Furthermore, the MVTR of such packaging materials is too low for use as a fresh meat wrap. Again, as noted above, a MVTR of from about 10 to 50 g/100 sq.in./24 hr. at 100.degree. F. and 90% R.H. is desirable to help prevent fogging of such wraps.